The present invention relates to a process for preparing water-soluble sodium carboxymethyl cellulose (NaCMC) by etherifying cellulose with alkyl monochloroacetates.
Water-soluble cellulose ethers have gained a wide variety of applications in practice due to their advantageous characteristics relating to, for example, viscosity behavior, water retention value, surface-activity and film-forming and binding properties. Among the most widely known cellulose ethers are the sodium carboxymethyl celluloses (NaCMC). The sodium carboxymethyl celluloses generally also include mixed cellulose ethers which, in addition to the ionic sodium carboxymethyl groups (NaCM), also comprise non-ionic substituents, such as methyl groups, hydroxyethyl groups and/or hydroxypropyl groups. Mixed cellulose ethers include, for example, sodium carboxymethyl methyl cellulose (NaCMMC), sodium carboxymethyl hydroxyethyl cellulose (NaCMHEC) and sodium carboxymethyl methyl hydroxypropyl cellulose (NaCMMHPC). Sodium carboxymethyl celluloses of these types are used, for example, in the following technical fields and products: production of detergents, oil drilling, mining, textile industry, paper manufacturing, adhesives and coating materials, foodstuffs, cosmetics and pharmaceutical preparations.
In the prior art, a number of continuous and discontinuous processes for the production of cellulose ethers are disclosed [see, e.g. "Ullmanns Encyklopaedie der technischen Chemie" - "Ullmann's Encyclopedia of Industrial chemistry" - Verlag Chemie, Weinheim, 4th edition, volume 9 ("Butadien" to "Cytostatika"), 1975, keyword "cellulose ethers", pages 203/204]. These processes are essentially based on two fundamental principles:
etherification is carried out in an aqueous-alkaline medium, or
etherification is carried out in an aqueous-alkaline medium which additionally comprises an organic solvent, e.g., isopropanol or tert.-butanol, (slurry, dispersion).
In the two process variants, free monochloroacetic acid or a salt thereof (usually sodium monochloroacetate) can be used as the etherifying agent. As alternatives for these etherifying agents, corresponding esters have also been disclosed.
According to German Offenlegungsschrift No. 2,062,245 (corresponding to U.S. Pat. No. 3,705,890) or German Auslegeschrift No. 2,556,754 (corresponding to U.S Pat. No. 4,097,667) cellulose or cellulose derivatives, for example, cellulose ethers, are reacted with chloroformic acid esters in the presence of a base, these esters reacting, as a rule, in a bi-functional manner and yielding water-insoluble reaction products.
In the process for preparing alkali carboxymethyl cellulose according to U.S. Pat. No. 3,900,463, cellulose is first caused to interact with an etherifying agent selected from the group consisting of alkali monochloroacetate and alkyl monochloroacetate, in a solvent system composed of water and at least one organic solvent, until the etherifying agent is uniformly distributed within the cellulose mass. The etherifying agent is used in an amount ranging from 0.4 to 2.0 moles per mole of cellulose. It is only after this primary mixing with the etherifying agent that alkalization, using not less than 2 moles of alkali hydroxide per mole of chloroacetate, and the actual etherifying reaction are carried out. Suitable organic solvents include alkanols, alkane-diols and alkane-triols having an alkyl group containing from 2 to 4 carbon atoms, alkoxy-alkanols having an alkoxy group containing from 2 to 4 carbon atoms, aromatic hydrocarbons or dialkyl ketones having alkyl groups contaning from 1 to 4 carbon atoms. The solvent system is used in an amount of not less than 2 parts by weight, preferably from 6 to 16 parts by weight, per part by weight of cellulose. In the first treatment step, the reaction time is about 1 h, at a temperature below 40.degree. C., followed by the alkalization step at approximately the same temperature and for the same duration. The actual etherifying reaction is carried out for several hours at 60.degree. to 80.degree. C. It is pointed out that the particular advantages resulting from the use of alkyl monochloroacetates, as compared with alkali monochloroacetates are a) better solubility of the alkyl monochloroacetates in organic solvents, b) higher reaction activity, c) improved selectivity of reaction, and d) smaller amounts of required organic solvents. The NaCMC types prepared with the esters used as the etherifying agents exhibit viscosities ranging between 106 and 180 mPa.s, as measured in a 1% strength aqueous solution, at DS values from 0.70 to 1.06 (as indicated in Examples 24 to 27).
When the procedure described in U.S. Pat. No. 3,900,463 is carried out in practice it is found, however, that subsequent alkalization gives rise to a loss in the yield of reacted etherifying agent, which may presumably be ascribed to the fact that side reactions between the alkyl monochloroacetate to be reacted and the alkalizing agent are more likely to occur in a subsequent alkalization than in the case of alkali cellulose being present before the etherifying step. In addition, it is impossible, according to the process of U.S. Pat. No. 3,900,463, to produce a NaCMC which has very high viscosities, for example, of 50,000 mPa.s, 100,000 mPa.s, or even higher, measured (according to Hoeppler) at 20.degree. C. in a 1.8% strength aqueous solution. It is similarly impossible to produce a NaCMC having very high viscosities according to any of the other above-mentioned methods of preparing NaCMC, nor can this be achieved in an experimental procedure operating with superposition of an inert gas, such as, for example, nitrogen.